1. Field of the Invention
The present invention relates to a method of measuring a transmittance of a reticle that is used in manufacture of a semiconductor device, and a projection exposure device (projection aligner) and a projection exposure method that are used in the measurement.
2. Description of the Related Art
In a projection exposure device, for example, a stepper, when a reticle is first used, the reticle is actually loaded in the device to be exposed to light from a mercury lamp serving as a light source, and (transmitted energy/incident energy) is calculated as a reticle transmittance. Sampling is carried out through the exposure at an equal interval over the entire reticle patterns. Then estimation on characteristics is required over the entire reticle viewed as a parent population. However the sampling tends to be uneven since it is difficult in practice to recognize what pattern is there on the reticle. For example, in the case of a pattern that is repeated at the same pitch as that of the sampling, the result calculated as the reticle transmittance is far from the real value. A correction function is activated in projection exposure devices when the amount of transmitting light increases, in order to cancel out the effect of lens expansion from heat that is generated by the increased exposure load. However, incorrect feedback leads to a focus shift, causing an increase in line width fluctuation and a failure in maintaining the rectangular resist profile, which deteriorates formation of a desired pattern. Shorts and opens consequently develop in the wiring pattern, thereby impairing quality.
Increasing the sampling count has therefore been tried in an attempt to capture the entire picture of the reticle as the parent population. However, the fear of uneven sampling remains in the case of equal interval measurement and, even if accurate measurement is somehow managed, the vast amount of time required in this approach impairs the productivity of the projection exposure device.
As described in the description of the related art, the problem of a mismatch between the result acquired as the reticle transmittance and the real value may occur when a pattern repeated at the same pitch as that of the sampling is included. In the case where a reticle transmittance that takes into account the characteristics of patterns is determined, for example, one method of determining the reticle transmittance, when a different shot has a different area as in Japanese Published Patent Application No. 2001-297961, involves conducting total light amount measurement on the entire reticle surface, storing data about a projected image, and calculating an actually exposed portion of the reticle from the stored data and from the opening/closing of a masking blade. A prerequisite for the calculation of an actually exposed portion of the reticle is that accurate total light amount measurement on the entire reticle surface can be conducted first. When a pattern repeated at the same pitch as that of the sampling is included, a result calculated as the reticle transmittance does not match reality, which makes it difficult to calculate an actually exposed portion of the reticle from the opening/closing of the masking blade.
Increasing the sampling count in an attempt to capture the entire picture of the reticle as the parent population is a way to measure the reticle transmittance accurately. Another method of determining the reticle transmittance is described in Japanese Published Patent Application No. H06-236838, in which a reticle is actually loaded in a projection exposure device to be exposed to light from a mercury lamp serving as a light source, and image data of a transfer image of a reticle pattern formed by the exposure is input to be used in the calculation of the reticle transmittance. However, both methods have a problem in that the huge sampling count requires a vast amount of time for measurement and accordingly impairs the productivity of the projection exposure device.